The use of traditional jet propulsion devices for marine craft is well known technology. Jet propulsion has many advantages over the simple propeller, particularly in terms of shallow water navigation and maneuverability, though jet propulsion energy consumption is much less efficient than traditional propeller systems. However, widespread acceptance of j et propulsion for marine craft has not occurred because of certain common problems associated with marine jet propulsion. For example, marine jet propulsion can pose significant design application issues because of uncertain performance over a wide range of speeds, water depth, sea conditions, excess water pickup at the jet propulsion unit inlet that may cause balling and others, etc.
Cavitation is another common problem. Cavitation represents an uneven pressure load (net positive suction head) on the impeller. Cavitation can be produced by excessive radial acceleration of the fluid, excess swirl and aerated turbulence of the fluid column, and pressure changes that cause unintentional partial vaporization of the fluid throughput associated with a vacuum produced by impeller action.
Accordingly, it would be desirable to design a jet like propulsion unit for marine vessels with the propulsion efficiency of a propeller where each feature synergistically works together to provide for a constant solid, unaerated column of water even at high output and where the water throughput is neither turbulent nor swirling in order to eliminate cavitation and pressure changes effects. Furthermore, the unit should have synergetic vessel application with maximum flexibility to cope with the entire speed range of the marine vessel and varied loading on the unit of its prime mover without producing the above-mentioned balling and cavitation effects.